The present invention relates to a system for capturing images of the face of an individual, a face identification system using such an image capture system, and an illumination device for such an image capture system. It finds application in the field of facial recognition of individuals.
A method of facial identification of an individual comprises a step of capturing at least one image of his face, a step of processing the image thus captured, a step of comparing the image thus processed with reference images stored in a database and a step of identification or non-identification of the individual using the result of the comparison step.
A face identification system implementing such a face identification method comprises a system for capturing images of the face of the individual and a processing unit.
The system for capturing images of the face of an individual of the prior art implementing the capture step comprises:                an image capture device, of the camera type, having a line of sight directed towards the place from where the individual arrives, and        two illumination devices disposed symmetrically with respect to the line of sight and at a distance therefrom, each comprising means for generating a light flux intended to illuminate the face of the individual as he moves and whereof the illumination lobe, representative of the luminance, has a maximum, the two lobes being symmetrical with respect to the line of sight.        
The processing unit implements the other steps of the identification method.
An individual wishing to have himself identified is asked to pass between the two illumination devices in order that the capture device can take at least one image of his face.
Such an identification method is more reliable the more the illumination of the face of the individual is powerful but non-saturated, homogenous and not asymmetric laterally and producing few shadows. Otherwise, the captured image has over-illuminated areas and under-illuminated areas depending on the position of the individual with respect to the line of sight.
FIG. 4 depicts a top view of an illumination device 400 of the prior art which comprises an opaque cage 408 with a window 410 made therein and a light source 402 of the neon type forming the light flux generation means and disposed inside the cage 408. Each illumination device emits, through the window 410, a light flux which is represented by an illumination lobe 406 representative of the luminance in a horizontal plane of the light flux. The straight line referenced 404 passes through the maximum and the illumination lobe 406 is symmetrical with respect to this straight line 404 which makes, with the line of sight 112, an angle α of the order of 50° in order to effectively illuminate the face of the individual and not his side.
An individual who is offset with respect to the line of sight 112 is, through the shape of each illumination lobe 406, strongly illuminated by the illumination device 400 situated on the side where the individual is located and weakly illuminated by the illumination device 400 situated on the other side, thus creating a large illumination disparity on the face of the individual. For a variation of approximately ±30° around the straight line 404, the luminance can be considered as constant. The illumination at a point is inversely proportional to the square of the distance between this point and the illumination device. An individual moving on one side of the line of sight will thus be more strongly illuminated by the illumination device on this side than by the one on the other side.
One solution to this problem consists of greatly constraining the position of the individual during image capture. Such a solution is not optimum on account of this constraint which may inconvenience the individual.
The document U.S. Pat. No. 5,550,928 discloses a recognition device making it possible to recognise individuals placed in front of a television screen and comprising an image capture system.
The capture system comprises:                two IR cameras, each comprising a line of sight and the two lines of sight being convergent with each other, the cameras being directed towards the place where the individuals are located, and        two illumination devices disposed symmetrically with respect to a central axis (which here is parallel to the element 44) of the device and at a distance therefrom, each comprising means of generating a light flux intended to illuminate the individuals and whereof the illumination lobe, representative of the luminance in a horizontal plane, has a maximum, the two lobes being symmetrical with each other with respect to the central axis.        
As can be seen in FIG. 2 of the document of the prior art and in the corresponding description, the illumination devices are oriented to favour a broad illumination of the space. In particular, through the configuration of each illumination device, each illuminates the space that is located in front of the capture system and the space that is located on the sides thereof.
Through the construction of each illumination device, the straight line bearing the maximum originates from each illumination device and is not oriented towards the central axis of the capture system but in a direction opposite to this central axis.
The use of two cameras makes it possible to observe the space that is illuminated under this broad angle and overall, each camera observes the area illuminated by one of the illumination devices and not the area illuminated by both illumination devices.
The recognition device of the document of the prior art therefore allows a broad illumination of the upstream space, but does not make it possible to focus the illumination towards a specific area of this space, which is desired in the case of an analysis of the face of an individual.
BRIEF SUMMARY OF THE INVENTION
One object of the present invention is to propose a system for capturing images of the face of an individual which does not have the drawbacks of the prior art, and which, in particular, permits a freer positioning of the individual whilst ensuring a homogeneous illumination of his face.
To that end, there is proposed a system for capturing images of the face of an individual comprising:                an image capture device having a line of sight directed towards the place from where the individual arrives, and        two illumination devices disposed symmetrically with respect to the line of sight and at a distance therefrom, each comprising means of generating a light flux intended to illuminate the face of the individual as he moves and whereof the illumination lobe, representative of the luminance in a horizontal plane, has a maximum borne by a straight line originating from each illumination device, the two lobes being symmetrical with each other with respect to the line of sight,        
the capture system being characterised in that the generation means are such that each lobe is asymmetric with respect to the straight line bearing said maximum and has, from this maximum, a first front curved towards the image capture device and a second front that is substantially plane and extending on the side from where the individual comes, and in that, for each illumination device, the straight line bearing the maximum is oriented towards the line of sight.
Advantageously, the image capture system comprises a third illumination device situated in the vicinity of the ground and comprising means of generating a light flux intended to illuminate the face of the individual as he moves and whereof the illumination lobe, representative of the luminance in the vertical plane, has a maximum and the light flux generation means are such that the lobe is asymmetric with respect to the straight line bearing said maximum and has, from this maximum, a first front that is curved and oriented towards the individual and a second front that is substantially plane and extending on the side of the image capture device.
Advantageously, the generation means comprise a first Lambertian source and a micro-prism film, the normal to the first Lambertian source and the normal to the micro-prism film making a non-zero angle with each other.
Advantageously, the generation means comprise a second Lambertian source, the normal to which makes a non-zero angle with the normal to the micro-prism film.
Advantageously, the normal to the first Lambertian source and the normal to the second Lambertian source are substantially orthogonal.
Advantageously, the micro-prism film comprises a plane face oriented towards the Lambertian source or sources and a face carrying prisms.
The invention also proposes a face identification system comprising an image capture system according to one of the preceding variants and a processing unit comprising means for processing the images captured by the image capture system, analysing said images thus processed and identifying each individual using said analysis.
The invention also proposes an illumination device comprising means of generating a light flux whereof the illumination lobe, representative of its luminance in a horizontal plane, has a maximum, and characterised in that the generation means are such that each lobe is asymmetric with respect to the straight line bearing said maximum and has, from this maximum, a first curved front and a second front that is substantially plane.
Advantageously, the generation means comprise a first Lambertian source and a micro-prism film, the normal to the first Lambertian source and the normal to the micro-prism film making a non-zero angle with each other.
Advantageously, the generation means comprise a second Lambertian source, the normal to which makes a non-zero angle with the normal to the micro-prism film.
Advantageously, the normal to the first Lambertian source and the normal to the second Lambertian source are substantially orthogonal.
Advantageously, the micro-prism film comprises a plane face oriented towards the Lambertian source or sources and a face carrying prisms.